Mechanical stresses in the heart are important determinants of regional physiology and pathophysiology. Stresses cannot be measured in the intact heart, however, but must be calculated via biomechanical analyses. Fundamental to these analyses is a complete description of the mechanical properties of the tissues, which is currently unavailable. Understanding the mechanical properties of myocardium require delineation of the active and passive components of the behavior. This is probably best accomplished by first characterizing the passive properties since it is well known (Frank-Starling mechanism) that the passive state is an important determinant of active myocardial behavior. Moreover, there is much speculation that passive myocardial properties vary regionally within the heart although there are no data to verify this. Consequently, the specific aims of this proposal are to perform the requisite experiments, and determine the stress-strain relations for normal canine epicardium and passive myocardium within different regions in the heart. This will be accomplished by (a) performing standard biaxial tests on epicardium, (b) identifying new biaxial protocols necessary to determine myocardial constitutive relations directly from data on metabolically inactivated tissues; (c) identifying new methods to measure biaxial strains; (d) establishing new experimental guidelines to ensure that the data is reliable; (e) using nonlinear regressions and nonparametric statistics to quantify the mechanical properties in terms of material parameters, and (f) developing a new model for evaluating the applicability of constitutive relations determined from excised specimens to the intact heart. In this way, many gaps can be filled with regard to our current knowledge of the passive mechanical properties of the tissues, the predicted distribution of stresses in the heart, and the effects of these stresses on cardiac performance. Moreover, this data will be fundamental to future investigations on active behavior and alterations in properties due to age, disease and medications.